1. Field of the Invention
The present invention relates to a light scanning apparatus, a light scanning method, an image forming apparatus, and a color image forming apparatus.
2. Description of the Related Art
FIG. 23 is a block diagram illustrating an exemplary configuration of an electrophotographic image forming apparatus such as a laser printer or a digital copier. Referring to FIG. 23, a laser beam emitted from a semiconductor laser unit 1001 used as a light source unit is deflected and scanned by a rotating polygon mirror 1002, passes through a scanning lens (fθ lens) 1003, and forms a light spot on a photoreceptor 1004. The photoreceptor 1004 is exposed to light, so that an electrostatic latent image is formed. A phase-locked loop 1009 synchronizes the phase of a modulation signal generated by a clock generation circuit 1008 with the phase of an output signal from a photodetector 1005 that detects the laser beam deflected by the polygon mirror 1002. More specifically, the phase-locked loop 1009 generates, for each line, an image clock (pixel clock) phase-synchronized with the output signal from the photodetector 1005 and supplies the image clock to an image processing unit 1006 and a laser driving circuit 1007. The laser driving circuit 1007 controls the laser emission time of the semiconductor laser unit 1001 according to image data generated by the image processing unit 1006 and the phase-synchronized image clock generated by the phase-locked loop 1009 for each line, thereby controlling the formation of an electrostatic latent image on the photoreceptor (to-be-scanned medium) 1004.
In recent years, increasing demands for higher printing speed (image forming speed) and better image quality have been satisfied by increasing the rotational speed of a polygon motor used as a deflector and the frequency of a pixel clock used as a reference clock for laser modulation. However, further improvements in printing speed and image quality would be difficult with such conventional methods.
For the above reason, multibeam technologies using plural light sources have been developed to achieve higher printing speed and better image quality. A light scanning method using a multibeam technology increases the number of light beams that can be deflected and scanned by a deflector at a time. Such a light scanning method enables lowering the rotational speed of a polygon motor used as a deflector and the frequency of a pixel clock, thereby making it possible to provide faster and more stable light scanning and image forming.
As a semiconductor laser unit or light sources for providing plural light beams in a multibeam technology, a combination of single-beam laser chips or an LD (laser diode) array having plural light-emitting elements on a single laser chip is being used.
Such a semiconductor laser unit, for example, an LD array, for providing plural light beams is very compact, can perform direct modulation at a very high speed using a driving current, and therefore has been widely used as light sources for a laser printer or the like in recent years. However, since the light output of a semiconductor laser at a driving current varies depending on the temperature, it is difficult to maintain the light intensity of a semiconductor laser at a specific level. Especially, in a surface emitting laser array having plural light sources on a single chip, since the distances between the light sources are short, temperature variation due to light emission and extinction and temperature cross-talk is likely to cause fluctuation of the amount of light emitted from the light sources.
Patent document 1 discloses a light scanning apparatus having a two-dimensional array of plural light sources that scans a photoreceptor by deflecting plural light beams with a deflector. According to an embodiment in patent document 1, the density of light emitting points can be maximized without causing temperature cross-talk between the light emitting points.
Patent document 2 discloses an image forming apparatus using surface emitting lasers. An embodiment in patent document 2 makes it possible to change light intensity of each laser chip pixel-by-pixel and to control the light emission time of each laser chip pixel-by-pixel, thereby allowing to control formation of an electrostatic latent image of pixels.
Patent document 3 discloses a light scanning apparatus using surface emitting lasers. An embodiment in patent document 3 obviates the problem of heat cross-talk using a specific arrangement of light sources and thereby makes it possible to form a high-density image.
<Patent Document 1> Japanese Patent Laid-Open Publication No. 2001-272615
<Patent Document 2> Japanese Patent Laid-Open Publication No. 2003-72135
<Patent Document 3> Japanese Patent Laid-Open Publication No. 2001-350111
However, in a conventional light scanning apparatus having plural light sources, since one pixel is normally formed with one light source, it is difficult to correct the position of a pixel with an accuracy higher than the size of the pixel.
In order to overcome such a problem, the applicant of the present invention has developed a method that causes M ((N−1)≧M≧1) light sources out of N (N≧−2) light sources, which can scan different positions in the sub scanning direction in a pixel formed on a photoreceptor, to emit light and scan, and thereby corrects the position of a pixel in the sub-scanning direction with an accuracy higher than the pixel density in the sub scanning direction. This method, however, is disadvantageous in that, in the case of providing M light sources with different data pieces, costs of drive units of the M light sources are high.
FIG. 50 is a block diagram illustrating an exemplary configuration of an electrophotographic image forming apparatus such as a laser printer or a digital copier. Referring to FIG. 50, a laser beam emitted from a semiconductor laser unit 31001 used as a light source unit is deflected and scanned by a rotating polygon mirror 31002, passes through a scanning lens (fθ lens) 31003, and forms a light spot on a photoreceptor 31004. The photoreceptor 31004 is exposed to light, so that an electrostatic latent image is formed. A phase-locked loop 31009 synchronizes the phase of a modulation signal generated by a clock generation circuit 31008 with the phase of an output signal from a photodetector 31005 that detects the laser beam deflected by the polygon mirror 31002. More specifically, the phase-locked loop 31009 generates, for each line, an image clock (pixel clock) phase-synchronized with the output signal from the photodetector 31005 and supplies the image clock to an image processing unit 31006 and a laser driving circuit 31007. The laser driving circuit 31007 controls the laser emission time of the semiconductor laser unit 31001 according to image data generated by the image processing unit 31006 and the phase-synchronized image clock generated by the phase-locked loop 31009 for each line, thereby controlling the formation of an electrostatic latent image on the photoreceptor (to-be-scanned medium) 31004.
As mentioned above, in recent years, increasing demands for higher printing speed (image forming speed) and better image quality have been satisfied by increasing the rotational speed of a polygon motor used as a deflector and the frequency of a pixel clock used as a reference clock for laser modulation. However, further improvements in printing speed and image quality would be difficult with such conventional methods.
For the above reason, multibeam technologies using plural light sources have been developed to achieve higher printing speed and better image quality. A light scanning method using a multibeam technology increases the number of light beams that can be deflected and scanned by a deflector at a time. Such a light scanning method enables lowering the rotational speed of a polygon motor used as a deflector and the frequency of a pixel clock, thereby making it possible to provide faster and more stable light scanning and image forming.
As a semiconductor laser unit or light sources for providing plural light beams in a multibeam technology, a combination of single-beam laser chips or an LD (laser diode) array having plural light-emitting elements on a single laser chip is being used.
<Patent Document 4> Japanese Patent Laid-Open Publication No. 7-276704
<Patent Document 5> Japanese Patent Laid-Open Publication No. 9-200431
In a typical system of light scanning apparatuses using a multibeam technology, one light source forms a one pixel, and hence a variation in light emitting level between the light sources directly leads to a variation in image density.
For forming one pixel by using one light source, since the light source needs to have light intensity high enough to generate a latent image, a high current is applied to a semiconductor laser. However, applying a high current to a semiconductor laser reduces the service life of the semiconductor laser.
To solve these problems, plural light sources each requiring less current and providing less amount of light may be used to form one pixel. The light sources are disposed at regular intervals at a density less than a pixel intensity at which one light source forms one pixel.
FIG. 24 is a diagram showing an example of forming one pixel by using plural light sources each of which requires less current and provides less amount of light. More specifically, N light sources (in FIG. 24, eight light sources) A through H are aligned in a sub scanning direction, and each light source group (in FIG. 24, each group of four light sources) forms one pixel. The term “light source group” as used herein indicates M light sources (in FIG. 24, four light sources) out of the N light sources, scanning positions of which M light sources are adjacent to each other in the sub scanning direction. That is, in the example shown in FIG. 24, among the eight light sources, a first light source group (the light sources A, B, C, and D) is used to form one pixel; and a second light source group (the light sources E, F, G, and H) is used to form one pixel.
In the example shown in FIG. 24, the N light sources are disposed at regular intervals (of distance X1). In other words, the interval between light sources in each light source group (each interval between the light sources A and B, between the light source B and the light source C, between the light source C and the light source D, between the light source E and the light source F, between the light source F and the light source G, and between the light source G and the light source H) is a distance X1, and the interval between the light source groups (the interval between the light source D and the light source E) is also the distance X1. With this configuration, as shown in FIG. 24, a spread of a light beam of each light source group of four light sources (spreading of light beams emitted from the light sources A, B, C, and D, and spreading of light beams emitted from the light sources E, F, G, and H) adversely affects a line (pixel) adjacent in the sub scanning direction.